It would be a separate dx. Hoarding is not a symptom of bipolar.

As others have mentioned (and what you wonder), mania can (though not always) include spending sprees where a bunch of stuff is accumulated. But as BirdDancer pointed out, those without a co-morbid hoarding issue would tend to return/get rid of/organize/whatever. Hoarding got tucked under the OCD/anxiety umbrella, though I remember reading that they were thinking of putting it as its own separate disorder (I don't know if that happened or not.)

Personally, I am the anti-hoarder, lol. My mother is a hoarder (she has multiple issues) and I'd be damned before I'd go down that path. I majorly purge possessions with ease. I live comfortably and spaciously in 250 sq ft.